Collaborative Research: Developing a Methodology for Imaging Stress Transients at Seismogenic Depth: Data Analysis and Interpretation

断层系统随时间变化的应力状态可能是控制地震事件序列和成核的最重要的单一属性。作为地球望远镜的一部分，地面大地测量仪器的部署将通过对时变的地表应变场的全面观测，为我们提供对这一应力场的重要约束。作为加强这些约束的一种方式，PIS目前正在开发一种基于地震速度的应力相关性的有源方法来测量地下应力瞬变。几十年来的大量实验室研究表明，地壳岩石的弹性性质(地震速度、衰减、各向异性)明显表现出应力相关性。这种相关性归因于微裂纹的张开/闭合，这是由于垂直于裂纹表面的应力的变化。因此，原则上可以通过利用孕震地壳弹性性质的应力敏感性来检测应力变化。几十年来，人们一直在努力利用这种压力依赖，尽管到目前为止，这一目标一直难以实现。这主要有两个原因：1)缺乏足够的时延精度来检测应力的微小变化，2)在应力和介质的地震性质之间建立可靠的校准是困难的。这两个问题是耦合在一起的，因为最好的校准源是固体固体潮和大气压，这两者都会产生102-103pA的弱应力扰动。探测这些震源需要在实验室实验的基础上测量10-5-10-6量级的速度变化分数。私营部门一直在进行一系列不同规模的跨孔有源源实验：劳伦斯·伯克利国家实验室设施的间距为3米，里士满野外站的间距为30米，从帕克菲尔德的SAFOD试验孔到SAFOD主孔的传感器的间距为300米，深度为2公里。到目前为止，他们已经完成了第一个地点的工作，在RFS进行了几次测量，并在RFS完成了最后的测试。对两个试验数据集的初步分析表明，有可能达到所需的10-6量级的延迟时间精度，并且可以观察到大气压和潮汐引起的旅行时间变化。利用RFS和Parkfield的最终数据集，PI计划结合数值模拟进行以下分析：(1)P波及其尾波的延迟时间估计；(2)S波及其尾波的延迟时间估计；(3)P波和S波的幅度测量；(4)S波的分裂测量；(5)P波和S波的散射场成像。数值模拟包括：1)确定相应孔隙弹性介质的特征及其对已知应力的响应；2)计算此类介质的相应地震属性。然后，最有希望的特性将被用来开发定量应力校准，通过选择适当的多孔弹性介质来建立，该介质既考虑了观测到的应力敏感性，又考虑了地震特性(包括散射)。

0453638SilverThe time-varying stress state of fault systems is perhaps the single most important property controlling the sequencing and nucleation of seismic events. The deployment of surface geodetic instrumentation, as part of Earthscope, will provide us with important constraints on this stress field, through comprehensive observations of the time-varying surface strain field. As a way of augmenting these constraints, the PIs are presently developing an active-source methodology based on the stress dependence of seismic velocity to measure subsurface stress transients. Numerous laboratory studies over several decades have shown that the elastic properties (seismic velocity, attenuation, anisotropy) of crustal rocks clearly exhibit stress dependence. Such dependence is attributed to the opening/closing of microcracks due to changes in the stress normal to the crack surface. Thus stress changes can, in principle, be detected by exploiting the stress sensitivity of the elastic properties of the seismogenic crust. For decades there have been efforts to exploit this stress dependence, although this goal has thus far been elusive. There are two primary reasons for this: 1) lack of sufficient time-delay precision necessary to detect small changes in stress, and 2) the difficulty in establishing a reliable calibration between stress and the seismic properties of the medium. These two problems are coupled because the best sources of calibration are the solid-earth tides and barometric pressure, both of which produce weak stress perturbations of order 102-103 Pa. Detecting these sources requires measurement of fractional velocity changes on the order of 10-5-10-6, based on laboratory experiments. The PIs have been conducting a series of cross-hole active-source experiments at different scales: 3 m spacing at the Lawrence Berkeley National Laboratory (LBNL) facility, 30 m spacing at the LBNL Richmond Field Station (RFS), and, 300 m spacing at 2 km depth, shooting from SAFOD Pilot Hole at Parkfield to sensors at the SAFOD main hole. Thus far they have completed work at the first site, have made several measurements at the RFS, and are completing the final test at RFS. Preliminary analyses of the two test datasets suggest that it is possible to reach the required delay time precision of order 10-6, and that barometric pressure and tidally-induced changes in travel time can be observed. With the final datasets from RFS and Parkfield, the PIs are planning to conduct the following analysis together with numerical modeling: (1) delay time estimates of the P wave and its coda; (2) delay time estimates of the S wave and its coda; (3) amplitude measurements for both P and S wave; (4) S-wave splitting measurements; (5) scattered-field imaging using P- and S-wave coda. Numerical modeling includes: 1) determining the characteristics of the corresponding poroelastic medium and its response to known stresses, and 2) calculating the corresponding seismic properties of such a medium. The most promising properties are then to be used to develop a quantitative stress calibration, established through the choice of an appropriate poroelastic medium that accounts for both the observed stress sensitivity and seismic properties (including scattering).